diff --git a/sys/arch/xen/xen/xbd_xenbus.c b/sys/arch/xen/xen/xbd_xenbus.c
index 59598022b16d..073ca9a4f6ab 100644
--- a/sys/arch/xen/xen/xbd_xenbus.c
+++ b/sys/arch/xen/xen/xbd_xenbus.c
@@ -1006,8 +1006,8 @@ xbd_diskstart(device_t self, struct buf *bp)
 
 	va = (vaddr_t)xbdreq->req_data & ~PAGE_MASK;
 	off = (vaddr_t)xbdreq->req_data & PAGE_MASK;
-	bcount = bp->b_bcount;
-	bp->b_resid = 0;
+	bcount = MIN(bp->b_bcount, XBD_MAX_XFER);
+	bp->b_resid = bp->b_bcount - bcount;
 	for (seg = 0; bcount > 0;) {
 		pmap_extract_ma(pmap_kernel(), va, &ma);
 		KASSERT((ma & (XEN_BSIZE - 1)) == 0);